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Abstract:

The audio reproduction system amplifies and reproduces both a digital
audio signal and an analog audio signal. After delta sigma modulation by
a ΔΣ modulator, the digital audio signal is pulse width
modulated by a first PWM converter and supplied to a selector. The analog
audio signal is compared with a PWM carrier signal by a second PWM
converter, converted to a PWM signal, and supplied to the selector. A
signal selected by the selector is then amplified by a class D amplifier.

Claims:

1. An audio reproduction system for amplifying and reproducing both a
digital audio signal and an analog audio signal, comprising:a delta sigma
modulator for performing delta sigma modulation on the digital audio
signal;a first PWM converter for converting an output of the delta sigma
modulator into a PWM signal by pulse width modulation;a second PWM
converter for comparing the analog audio signal and a PWM carrier signal
and converting to a PWM signal;a selector for selecting and outputting
either output of said first PWM converter or said second PWM converter;
anda class D amplifier for receiving and amplifying the output from said
selector.

2. A system according to claim 1, wherein:said selector selects either
output of said first PWM converter or said second PWM converter in
accordance with a control signal from an external source.

3. A system according to claim 1, wherein:said first PWM converter
determines the duty ratio of said PWM signal in accordance with a value
of the output of said delta sigma modulator.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001]The disclosure of Japanese Patent Application No. 2008-070612 filed
on Mar. 19, 2008, including specification, claims, drawings, and
abstract, is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to an audio reproduction system for
amplifying and reproducing both digital and analog audio signals.

[0004]2. Description of the Related Art

[0005]Heretofore, the two types of audio signal reproduction include the
reproduction of digital signals, such as from CDs and DVDs, and the
reproduction of analog signals, such as from AM and FM radio. Therefore,
in audio reproduction systems, it is necessary to have the capability of
reproduction for inputs for both digital and analog. For digital audio
signals in this type of system, particularly in portable devices, PWM
signals are often amplified by a class D amplifier after delta sigma
(ΔΣ) modulation. The reason for this is class D amplifiers
have good power efficiency and it is desirable for the audio reproduction
system to have the lowest possible power consumption in portable devices
where battery life becomes a problem. Thus, after ΔΣ
modulation by a ΔΣ modulator, digital audio signals obtained
from playback of a CD or DVD are converted to PWM signals, and then
amplified by a class D amplifier. On the other hand, for analog audio
signals, the signals are amplified by a class AB amplifier.

[0006]Furthermore, Japanese Patent Laid-Open Publication No. 2006-333437
(referred to hereinafter as Patent Document 1) discloses a configuration
for converting an analog audio signal to a digital signal with an ADC
circuit and supplying this digital signal to a class D amplifier via a
PWM converter, which performs PWM conversion for digital audio signals.
According to this configuration, the class AB amplifier can be omitted.

SUMMARY OF THE INVENTION

[0007]However, in the circuit of Patent Document 1, a digital signal
obtained by A/D conversion from an analog audio signal and a digital
audio signal are supplied to a single PWM converter. The digital audio
signal is generally a digital signal that has undergone PCM conversion.
Therefore, to obtain such a digital signal, after ΔΣ
modulation, it is necessary to process the analog audio signal with a
digital filter. Consequently, a combination of a ΔΣ modulator
and a digital filter becomes necessary in the ADC circuit resulting in a
problem where the circuit configuration becomes comparatively large.

[0008]The present invention is an audio reproduction system for amplifying
and reproducing both a digital audio signal and an analog audio signal
and includes a delta sigma modulator for performing delta sigma
modulation on the digital audio signal, a first PWM converter for
converting an output of the delta sigma modulator into a PWM signal by
pulse width modulation, a second PWM converter for comparing the analog
audio signal and a PWM carrier signal and converting into a PWM signal, a
selector for selecting and outputting either output of the first PWM
converter or the second PWM converter, and a class D amplifier for
receiving and amplifying the output of the selector.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]FIG. 1 is a block diagram showing a configuration of an embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010]The embodiment of the present invention will be described with
reference to the attached drawing.

[0011]FIG. 1 is a block diagram showing a configuration of an embodiment.
A reproduction apparatus of the embodiment performs amplification of both
a reproduced digital audio signal, such as that obtained from a CD or
DVD, and a reproduced analog audio signal, such as that obtained from FM
radio. Reproduction of both signals cannot be performed simultaneously
and one is selected so that either the digital audio signal or the analog
audio signal is input.

[0012]For example, a 44.1 kHz, 16-bit digital audio signal obtained from
the reproduction of a CD is input by a delta sigma (ΔΣ)
modulator 10. The ΔΣ modulator 10 performs delta sigma
(ΔΣ) modulation on the input digital audio signal for
conversion to a one-bit or multi-bit signal at a higher predetermined
frequency.

[0013]Then, this one-bit or multi-bit signal is converted to a PWM signal
at a first PWM converter 12. Namely, the PWM signal is generated with a
duty determined according to a value of the one-bit or multi-bit signal
at a frequency of the PWM carrier signal. The PWM signal is then supplied
to a selector 14.

[0014]Furthermore, the analog audio signal, such as from a radio tuner, is
supplied to a second PWM converter 16 where it is directly compared with
a PWM carrier signal, such as a triangular wave, so as to yield a PWM
signal with a duty ratio determined according to the comparison result
showing which of the PWM carrier signal or the analog audio signal is
larger. The PWM signal is then supplied to the selector 14.

[0015]The input of either the digital audio signal or the analog audio
signal is determined by a selection signal, such as from a microprocessor
for controlling the overall audio set. Consequently, the selection signal
controls the selector 14 so that the signal to be input is selected.

[0016]It is preferable to use the selection signal to terminate the power
supply to either the combination of the ΔΣ modulator 10 and
the first PWM converter 12 or the second PWM converter 16, whichever is
unused. 5 The PWM signal that was selected by the selector 14 is supplied
to a class D amplifier 18 where the signal is amplified, converted to an
analog signal, and then output. The analog signal is then supplied to a
speaker 20 from which sound is output according to the signal so that the
input signal is reproduced.

[0017]In this manner, according to the embodiment, either the digital
audio signal or the analog audio signal is amplified by the single class
D amplifier 18. Therefore, the power efficiency increases and the power
consumption can also be controlled to a low level. Furthermore, a circuit
such as the embodiment is generally formed from an integrated circuit
(IC) and the surface area of the IC and the number of components can be
reduced. For the digital audio signal, the combination of the
ΔΣ modulator 10 and the first PWM converter 12 is generally
employed to yield the PWM signal. For the analog audio signal, the signal
is directly converted to the PWM signal at the second PWM converter 16.
Therefore, an A/D converter including a ΔΣ converter and a
digital filter as shown in Patent Document 1 is not required and the
overall circuit size can be made smaller.

[0018]While there has been described what are at present considered to be
preferred embodiments of the invention, it will be understood that
various modifications may be made thereto, and it is intended that the
appended claims cover all such modifications as fall within the true
spirit and scope of the invention.